It is well known that color images can be obtained by exposing a silver halide color photographic light-sensitive material to light, followed by a color development during which the reaction between an oxidized aromatic primary amine developing agent and a dye forming coupler takes place. In such a process, the subtractive color process is usually utilized for color reproduction, and cyan, magenta and yellow color images, which are complementary to red, green and blue colors, respectively, are formed. The reaction between a coupler and an oxidation product of a color developing agent proceeds at an active point of the coupler. A coupler having a hydrogen atom at its active point (i.e., 4-equivalent coupler) stoichiometrically requires as an oxidizing agent 4 moles of silver halide having a development center for forming 1 mole of dye through the coupling reaction. On the contrary, a coupler having at its active point a group capable of being released in the form of an anion requires only 2 moles of silver halides having a development center for forming 1 mole of dye (i.e., 2-equivalent coupler). Accordingly, by using a 2-equivalent coupler, the amount of silver halide to be used in a light-sensitive layer can be reduced and the layer per se can be made thinner, so that the time required for the processing of such a light-sensitive material can be shortened and color images obtained therefrom can have an improved sharpness, compared with a light-sensitive material in which a 4-equivalent coupler is used. In addition, the coupling activity of a 2-equivalent coupler with a color developing agent can be widely varied, depending on the property of the releasable group contained therein.
A 2-equivalent coupler capable of releasing a group having a development-inhibiting effect is known and called a development inhibitor releasing coupler (or DIR coupler). Such couplers are capable of inhibiting development in proportion to the quantity of developed silver and, therefore, can be effective for improving graininess, controlling gradation and improving color reproducibility of the image. Couplers of this type can also be used in diffusion transfer processes, wherein their effects upon adjacent layers are utilized.
A 2-equivalent coupler can also be provided with a releasable group containing a diffusible dye portion. This class of couplers, which are referred to as diffusible dye-releasing coupler, can be utilized in a diffusion transfer process in which a dye image is formed from diffused dyes in an image receiving layer.
A certain colored 2-equivalent coupler can also be used to attain a masking effect necessary for color correction of dye images. This class of couplers are referred to as colored coupler.
As mentioned hereinbefore, 2-equivalent couplers can be provided with various functions, depending on the selection of releasable groups contained therein.
On the other hand, regarding the recent trend in the development of silver halide photographic light-sensitive materials, in particular, used for photographing, two major streams can be seen. One is the increase in sensitivity, as typically shown by ASA 400 films, etc. The other is the improvement in image quality to cope with the miniaturization of film sizes. In connection with the former, investigations have been made on a variety of techniques, including, e.g., large size silver halide grains, couplers with higher activities, acceleration of development, etc. However, the increase in sensitivity based on large size silver halide grains seems to be reaching its limit, as reported by G. C. Farnell and J. B. Chanter in J. Photogr. Sci., 9, 75 (1961). Accordingly, this technique is not expected to make much contribution in the future. In addition, the use of large size silver halide grains is accompanied by various disadvantages, such as deterioration in graininess. Couplers having higher activities have also been studied extensively. Such couplers, however, have not made much contribution to sensitivities of silver halide photographic light-sensitive materials, and proved to be disadvantageous for graininess. With regard to acceleration of development, various development accelerators, including hydrazine compounds, have hitherto been attempted in a silver halide emulsion layer or a developing solution therefor mainly with respect to black-and-white photographic light-sensitive materials. However, is most cases, the incorporation of development accelerators into an emulsion layer or a developing solution is accompanied by such disadvantages as increase in fog and deterioration in graininess and hence impractical.
Couplers which imagewise release development accelerators or fogging agents have also been proposed. For example, couplers releasing thiocyanic acid ions which accelerate the solution physical development are disclosed in U.S. Pat. Nos. 3,214,377 and 3,253,924, Japanese Patent Application (OPI) No. 17437/76, etc. Further, couplers releasing acyl hydrazines are described in Japanese Patent Application (OPI) No. 150845/82 and couplers releasing hydroquinone or aminophenol developing agents are described in Japanese Patent Application (OPI) No. 138636/82. However, the development accelerating function or the fogging function of these releasable groups is small and thus the effects can be obtained only when these couplers are used in a large amount. Also, the effects obtained are very small even if the couplers are employed in a large amount. Further, these couplers have the disadvantage in that color mixing occurs and the color reproducibility is degraded and in that the graininess is deteriorated since the releasable groups diffuse into other layers to cause development acceleration or to form fog due to their large diffusibility, when they are incorporated into a certain light-sensitive layer which is sensitive to light of a specific region in the spectrum. Moreover, most of these couplers have low stability and they undesirably cause several deteriorations in photographic properties, for example, increase in fog, desensitization, color contamination, etc., during preservation when they are incorporated into silver halide photographic light-sensitive materials.